This invention relates to a fuel control system for an internal combustion engine, and in particular to a fuel control system for an internal combustion engine wherein a true inlet air flow rate is detected for the engine control by taking account of a blow-back air flow rate.
Numerous systems for detecting the inlet air flow rate of an internal combustion engine for an automobile have been proposed. These have included a vane system for detecting the inlet air flow rate by detecting the dynamic air pressure based on the inclination of a movable vane, a system using von Karman's vortex street for detecting the inlet air flow rate based on the frequency of the vortex, and a hot-wire system for detecting the inlet air flow rate by utilizing the variation of the resistance of the hot-wire.
However, those inlet air flow rate detection systems have encountered pulsation and blow-back of the inlet air near the full air inlet condition where the throttle valve is fully opened at low engine speeds. This blow-back is caused be counter air flow from a cylinder of the engine to the inlet pipe when operation of the inlet valve and the exhaust valve of the engine overlap, as shown by the hatched portion in FIG. 1B which depicts air flow rate in relation to the motion of each of the pistons in the cylinders shown in FIG. 1A. Once the exhaust valve has closed and only the inlet valve is open, the air flow again is sucked into the engine. Due to this counter air flow, the true air flow rate can not be detected. Therefore, inlet air flow rate detection systems such as discussed above are easily influenced, depending upon the timing of the valves or the contour of the inlet pipe, when the engine speed is below 2000 rpm and the pressure in the inlet pipe is above -50 mmHg with respect to the atmosphere. Since a hot-wire system generates a signal independent of the direction of the air flow, it is particularly hard to distinguish blow-back air flow rate from air flow rate at the time of suction in these systems.
Other conventional fuel control systems for internal combustion engines utilizing a preset blow-back period according to the crank angle of the engine, or a thermal sensor system for detecting the direction of the air flow, as disclosed in Japanese Patent Application Laid-open No. 56-108909, and corresponding U.S. Pat. No. 4,404,846, have been proposed. However, both of these fuel control systems are complicated and expensive.